feat: add internal stats flush API

internal/middleware/internal_auth.go

@@ -0,0 +1,26 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+func InternalAuthMiddleware(expectedToken string) gin.HandlerFunc {
+	expectedToken = strings.TrimSpace(expectedToken)
+	return func(c *gin.Context) {
+		if expectedToken == "" {
+			c.Next()
+			return
+		}
+
+		token := strings.TrimSpace(c.GetHeader("X-Internal-Token"))
+		if token == "" || token != expectedToken {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid internal token"})
+			c.Abort()
+			return
+		}
+		c.Next()
+	}
+}